The present invention relates to a position measuring device, which is suited for measuring relative movements and, in addition, includes the feature of enabling an absolute reference to be established when positional determinations are made.
As a rule, conventional interferometers, used as position measuring devices to determine the position of parts that are movable relatively to one another, merely permit relative measurements. This means, for example, that in the case of a linear movement beginning from a starting point, the travel path covered is able to be defined in fractions of the particular interferometer wavelength. In practical applications, however, when, for instance, interferometers are used as integrated measuring systems in machine tools, it is sometimes beneficial in making the positional determination to be able to establish an absolute reference to a defined location of a known position within the measuring area. When conventional incremental measuring systems, which are based on the scanning of a periodic scale graduation, are used, the absolute reference is usually established on the basis of socalled reference markings placed at known positions on the scale side. Widely varying reference markings of this kind are known, such as so-called spacing-encoded reference markings, or also reference markings at defined locations. In each case, the reference markings are scanned to produce reference pulse signals, capable of being further processed by a downstream evaluation unit.
European Patent No. EP 0 305 438 B1 and German Patent Application No. DE 195 06 954 A1 propose linking an optical scanning unit to the movable measuring reflector of an incremental-measuring interferometer to scan a scale graduation arranged in parallel with the measuring distance in a generally known way. Reference markings are placed on the scale at defined positions. In this manner, even when working with incremental-measuring interferometers, which are used to measure the position of two parts that are movable in relation to one another, an absolute reference can be established during the positional measurement.
The drawback of these variants is that the interferometer beam components in the optical reference and measuring paths at the reference position in question cover different optical paths and, accordingly, are also variably affected by fluctuations in the ambient environment. Moreover, the scale graduation expands as a function of temperature, the influence of temperature on the scale graduation expansion being substantially greater than on the laser wavelength being used as a measuring standard. Thus, there is no constant relation between the temperature-dependent characteristics of the scale graduation and those of the laser wavelength. Therefore, when working with these means, determination of a particular absolute position is encumbered by environmentally related inaccuracies.
A further drawback of these proposed variants is that one must always ensure that the signal lines, which connect the scanning unit that is movable in the measuring direction to a downstream signal-processing unit, are securely routed in the measuring operation. This requires costly cable routing systems.
In the context of producing reference pulse signals when working with incremental-measuring interferometers, reference is also made to Japanese Patent Application No. JP 8-5767 and to U.S. Pat. No. 4,191,916. Both documents describe producing a reference pulse signal at a defined position along the measuring path with the aid of an optical switch. A disadvantage of producing a reference pulse signal in this manner is the fact that the absolute reference can only be established in comparison to the highly resolving, relative interferometer measurement with comparatively low accuracy.
An object of the present invention is to provide a method for a relative measuring position measuring device, in particular an interferometer, to produce a highly resolving reference pulse signal at at least one defined absolute position. In this context, a simplest possible design for the corresponding device is desirable, as is adequate stability of the measurement with respect to any fluctuations in the environmental conditions in the measuring area.
The present invention provides a position measuring device for determining the position of two parts that are movable relatively to one another in measuring direction (x), comprising
a light source (1);
beam-splitter element (2), which splits the beam of rays (10) supplied by the light source (1) into at least one first and one second beam component (10R, 10M), which are preferably oriented parallel to one another after leaving the beam-splitter element (2);
a reference reflector (3), arranged in a stationary mount in a reference path of rays, upon which the first of the two beam components (10R) strikes;
a measuring reflector (4), which is arranged in a measuring path of rays and which is movable in measuring direction (x) and upon which the second of the two beam components (10M) strikes;
at least one reference marking (62), capable of being scanned by a scanning unit (5), being arranged at the reference reflector (3) or at the measuring reflector (4) to produce a reference pulse signal at the location of the reference reflector (3); and the scanning unit (5) being arranged at the other reflector (3, 4), at the measuring reflector (4) or at the reference reflector (3).
Advantageous specific embodiments of the position measuring device according to the present invention include that: (a) the reference marking (62) is arranged at the particular reflector (3, 4) in such a way that a reference pulse signal is able to be produced via the scanning unit (5) arranged at the other reflector (3, 4), in response to identical optical path lengths in the optical measuring and reference path; (b) the reference marking (62) is arranged at the movable measuring reflector (4), and the scanning unit (5) is arranged at the stationary reference reflector (3); (c) the beam-splitter element (2) being designed as a prism; (d) the beam-splitter element (2) is designed as a double-grating beam splitter; (e) the measuring reflector (4) and the reference reflector (3) each are designed as triple prisms; (f) the scanning unit (5) is designed as an optical scanning unit, which includes a scanning radiation source (51), a scanning grating (53), as well as at least one scanning detector element (55, 56), by way of which the optical scanning of at least one reference marking (62) is possible; (g) the scanning unit (5) is designed as an optical scanning unit, which is able to be supplied with light from an external scanning radiation source and which includes a scanning grating, as well as at least one scanning detector element; (h) a laser, which is also provided as the light source (1) of the position measuring device, is used as an external scanning radiation source; (i) the reference marking (62) is arranged in a reference marking unit (6), which, in addition, includes an incremental graduation (61), and scanning detector elements (56) are arranged at the scanning unit (5) to produce incremental signals indicative of the relative movement of the reference and measuring reflector (3, 4) from the optical scanning of the incremental graduation (61); (j) the reference marking (62) is designed as a field having subregions with different optical properties configured in an alternating arrangement in a reference marking track (63), next to which is arranged adjacently, in parallel, an incremental graduation (61) in an incremental graduation track (65), which is made up of a periodic sequence of subregions having different optical properties; (k) the scanning unit (5) is arranged detachably at the particular intended reflector (3, 4); (1) the reference marking (62) is arranged in a reference-marking unit (6), which is mounted, in turn, detachably at the particular intended reflector (3, 4); (m) the scanning unit (5) and/or the reference-marking unit (6) is arranged so as to be adjustable with respect to position at the reference or measuring reflector (3, 4); (n) the output signals from the scanning unit (5) are able to be supplied via a signal-transmission line (20) to a downstream signal-processing unit (9); and /or (o) reference reflector (3) is arranged at the desired absolute position along the measuring path.
The present invention provides for using a suitable beam-splitter element to split a beam of rays supplied by a light source into two preferably parallel beam components for the optical measuring and reference path of an interferometer. Arranged in a stationary mount in the optical reference path is a reference reflector, to which is connected in turn a scanning unit, used, inter alia, to produce a reference pulse signal. In this case, the reference reflector is preferably arranged at that position along the measuring path which is to be determined as the absolute position, thus, for example, at a machine zero point. Located in the optical measuring path is the measuring reflector which is movable in the measuring direction and upon which is arranged at least one reference marking. Thus, in one particularly advantageous specific embodiment, a reference pulse signal is produced via the scanning unit mounted at the reference reflector, in response to identical optical path lengths in the optical measuring and reference path. In addition to the positional signals produced by the interferometer with respect to the relative movement, the resulting output signals from the scanning unit are fed to a signal-processing unit for further processing.
As a general principle, the scanning unit and the reference marking can also be arranged on a different reflector as the case may be.
At this point, the present invention ensures that in the case of the absolute position to be detected, the optical measuring and reference paths of the interferometer are always subject to the same environmental conditions. Thus, at this position of the movable measuring reflector, which needs to be determined with the highest possible precision, one can avoid any disparate influencing of the two optical paths. One can even improve upon this by selecting a suitable beam splitter element, thereby ensuring that the optical paths traversed in the particular beam-splitter element are as precisely identical as possible, and that the two optical path components run in parallel, spaced apart to the smallest extent possible.
Moreover, in one advantageous specific embodiment, it proves to be beneficial to mount a scanning unit on the stationary reference reflector to ultimately produce the reference pulse signal. This makes it possible to avoid the costs of routing cables to a scanning unit that is movable in the measuring direction. Thus, only once does one need to select the routing of the requisite signal lines from the scanning unit to the downstream signal-processing unit.
In connection with the selected scanning of the reference marking and the embodiment of the corresponding scanning unit, the most widely varying possibilities exist, of course, within the framework of the present invention. Thus, besides optical or photoelectric scanning, other physical scanning principles can also be applied. In the same way, very different interferometer variants can also be used in conjunction with the position measuring device of the present invention, for example homodyne interferometers, heterodyne interferometers, etc.